1. Field of the Invention
The present invention relates to an apparatus for measuring the quantity of AC electricity to derive the magnitude of the quantity of AC analog electricity on the basis of signals obtained through periodic sampling of a predetermined frequency.
2. Description of the Prior Art
An apparatus of this kind of the prior art, such as shown in FIG. 1 is introduced in "Hogo Keiden Kogaku" (Protective Relay Engineering), Inst. of Electrical Engineers of Japan, 112-113 (Jul. 20, 1981). In FIG. 1, indicated at (a) is AC voltage, at (b) is sampling time at which the AC voltage (a) is sampled and converted into a digital value, and at (c), (d) and (e) are sampling times within a time interval corresponding to the phase angle of the AC voltage. The period T=30.degree. . When the AC voltage was calculated on the basis of the present AC voltage V.sub.0 and the AC voltage V.sub.3 of 90.degree. before (3T=90.degree. ), at the sampling time (c) by the use of an expression: ##EQU1## the AC voltage was measured at an accuracy corresponding to 4-phase full-wave rectification. As well known, this accuracy is .+-.5.5%, which corresponds to ripple.
According to another well-known means to measure the AC voltage at a higher accuracy, the AC voltages V.sub.1 and V4 at the sampling time (d) and the AC voltages V.sub.2 and V.sub.5 at the sampling time (e) are used additionally and an operation similar to the Exp. (1) is executed. In this case, the accuracy is .+-.0.6%.
Since the conventional digital measuring apparatus has the above-mentioned constitution, on principle, the accuracy is deteriorated if the length of time corresponding to the phase angle 90.degree. the AC voltage is not the integral multiple of the sam period T. Accordingly, even if the sampling period is selected correctly, the measuring error can increase when the frequency of the AC voltage varies.
Furthermore, in case the AC voltage has changed suddenly, the method of FIG. 1 needs a time corresponding to 3T+3T=6T=180.degree. to detect a correct value, which is quite unsatisfactory in respect of following-up performance.